Radiation Protection in Space and on the Moon

By: Harrison Bralower

• Introduction
• Problem of radiation on the Moon
• Radiation from the sun, distant galaxies, etc. constantly strikes the Moon and the Earth’s magnetosphere doesn’t provide as much protection as you would think.
• Some can be worse than none¹
• High-energy particles can strike shielding and produce secondary radiation showers. This means that even more shielding is required to stop the secondary radiation.
• Electronics and radiation²
• Smaller circuits with digital and logical components are more vulnerable to radiation than other circuits.
• Stray radiation can cause logical errors and ruin hardware by ionizing components and charging electronic pieces.
• Water containment
• Water is vital to sustain life on the Moon, but currently there is no place to put it. Even if there were, the containment unit would need to be shielded against radiation to prevent the following reaction from taking place: 2H₂O_(l) à 2H_2(g) + O_2(g)
• Moon regolith
• Regolith has the potential to provide radiation shielding on the Moon, and depending on how it’s used it could provide shielding on trips to and from the Moon.
• Unkind conditions for equipment in space
• Geomagnetic storms³
• A temporary disturbance of Earth’s magnetosphere
• Disrupts communication (ground-to-air) and navigation systems (GPS, LORAN, OMEGA)
• Causes equipment malfunctions
• Differential charging: Different portions of a spacecraft obtain different charges, leading to harmful arcing
• Bulk/deep charging: High-speed electrons deposit charge internally after striking the outside of a spacecraft. Components then try to discharge to other components, which can disable entire systems.
• Solar wind⁴
• Solar wind carries Sun’s magnetic field lines across solar system
• Could cause induction problems on the Moon
• Magentohydrodynamic (MHD) dynamo
• Interplanetary Coronal Mass Ejections (ICMEs) could be a problem
• ICMEs can send electromagnetic waves and large magnetic fields towards earth
• Preceded by showers of ionizing radiation at the heliopause
• Van Allen belts/South Atlantic Anomaly^{2, 5}
• High radiation doses in these areas, especially in the Anomaly
• Satellites, etc. likely need to turn off their sensors in these areas
• Ring current⁶
• Ring current present in Earth’s equatorial plane
• May charge components or induce magnetic fields
• Brings up requirement for magnetic shielding on all components
• Cosmic radiation^{7, 8}
• Cosmic radiation is harmful to humans in space
• Cosmic radiation may also harm electronic components
• Launching events from the Earth may have to be timed with Coronal Mass Ejection (CME) events to take advantage of the Forbush decrease
• Problems equipment may encounter⁹
• SGEMP (Systems Generated Electromagnetic Pulse)
• Caused by radiation traveling through equipment
• Creates local ionization and induces currents in components
• SEE (Single-event Effects)
• Only affects digital devices
• SEU/MBU/SEFI (Single-event Upset/Multiple Bit Upset/Single-event Functional Interrupt)
• Change in memory state caused by single ion interaction
• MBU occurs in several adjacent memory cells
• SEU becomes SEFI if it places the device into an undefined state
• SEL (Single-event Latchup)
• Requires a chip with parasitic PNPN structure
• Heavy particle jams thyristor structure open
• Power cycling occurs
• Destructively high currents build until part fails
• Bulk CMOS components are most susceptible
• SET (Single-event Transient)
• Ionization event discharges into a false signal
• SES (Single-event Snapback)
• Similar to SEL, but without PNPN structure
• Induced in N-channel MOS transistors that switch large currents
• Transistor drain junction forced open and stays open
• SEB (Single-event Induced Burnout)
• Occurs in power MOSFETs
• Substrate under source region becomes forward-biased
• Drain-source voltage higher than breakdown voltage
• High currents and overheating can destroy the device
• SEGR (Single-event Gate Rupture)
• Occurs in power MOSFETs and EEPROM cells
• Occurs when a heavy ion hits the gate region while a high voltage is applied
• SiO₂ layer breaks down and the transistor gate explodes
• In EEPROM cells, it can happen during write/erase cycles when cells are subject to a high voltage
• Radiation Hardening⁹
• Process of making electronics impervious to radiation and its effects
• Different ways to do it
• Insulating substrates
• Used instead of traditional semiconductor wafers
• SOI (Silicon Oxide) and SOS (Silicon on Sapphire) are used
• Package shielding
• Use of SRAM (Static Random Access Memory) instead of DRAM (Dynamic Random Access Memory)
• Use of substrate with wide band gap
• Higher tolerance against deep-level defects
• Use of depleted B-11 in borophosphosilicate glass protecting layer
• Error correcting memory/scrubber circuits
• Additional parity bits check for corrupted data
• Sweeping RAM and checking for errors
• Redundant elements
• Three separate boards
• Independent answer computation
• Results compared; minority results recalculate
• Boards that give repeated minority results will shut down
• Three bits and voting logic
• Reserved for smaller designs
• Fail-safe in real time
• Produces correct results without resorting to watchdog timer
• Watchdog timer
• Last resort method
• Forces hard reset to the system unless a certain operation is performed (like a write cycle) routinely
• Water containment on the Moon
• Water, as an asymmetrical molecule, is susceptible to radiation
• Radiation splits water into hydrogen and oxygen gases
• Equilibrium between decomposition and recombination of water established by neutron and gamma radiation¹⁰
• Useful under certain circumstances
• Hydrogen and Oxygen are vital for life on the Moon
• The problem: Prevent water from dissociating or becoming irradiated while using a radiation shielding that isn’t hazardous to humans
• Heavier atoms block radiation better
• Lead, a typical “heavy” shield, is poisonous
• Hydrogen (esp. liquid hydrogen) is the best shield¹⁰
• It’s also very impractical to take to the Moon
• Polyethylene is a great substitute¹¹
• It has a lot of hydrogen—more than most polymers
• The solution: Make bricks out of a combination of polyethylene and Moon regolith to shielding buildings, water containment facilities, etc.¹¹
• Don’t have to take as much weight to the Moon
• Regolith makes an excellent filler material for radiation shields¹¹
• Readily available on the Moon
• Has few shielding properties of its own
• Conclusion
• Electronics require special attention in space
• Radiation hardening can prevent disastrous effects
• Water containment is possible in space
• Containment facilities need ample shielding
• Facilities also likely need hydrogen and oxygen collection equipment in case stray radiation comes through
• Moon regolith
• Much in-situ radiation shielding can be made out of polyethylene/regolith bricks
• Concern over transportation of heating equipment, drying ovens, steel molds
• Assuming proper precautions are taken, radiation shouldn’t be a problem

Footnotes

¹ “Ionising radiation protection” http://en.wikipedia.org/wiki/Ionising_radiation_protection

² “Van Allen radiation belt”

http://en.wikipedia.org/wiki/Van_Allen_Radiation_Belt

³ “Geomagnetic storm”

http://en.wikipedia.org/wiki/Geomagnetic_storm

⁴ “Solar wind”

http://en.wikipedia.org/wiki/Solar_wind

⁵ “South Atlantic Anomaly”

http://en.wikipedia.org/wiki/South_Atlantic_Anomaly

⁶ “Ring current”

http://en.wikipedia.org/wiki/Ring_current

⁷ “Cosmic ray”

http://en.wikipedia.org/wiki/Cosmic_ray

⁸ “Forbush decrease”

http://en.wikipedia.org/wiki/Forbush_decrease

⁹ “Radiation hardening”

http://en.wikipedia.org/wiki/Radiation_hardening

¹⁰ “Effects of Radiation on Water Chemistry (Synthesis)”

http://www.tpub.com/content/doe/h1015v2/css/h1015v2_22.htm

¹¹ “Researchers developing bricks to block radiation on Mars”

http://archives.cnn.com/2000/TECH/space/09/08/mars.bars.ap/index.html